Following DNA damage or inhibition of DNA synthesis, Escherichia coli exhibits a variety of new processes, such as prophage induction, new repair capacity, and mutagenesis. The products of two genes, recA and lexA, are required for expression of these processes. The proposed work is aimed at identifying the product of the lexA gene, purifying it, and determining its function. We are cloning the lexA gene onto plasmids and phages to achieve these aims. We intend to test the model that the lexA gene product is a repressor of the recA gene, as well as other models which involve the lexA gene in the regulation of the recA gene.